Permeation instruments are used to measure the transmission rate of a target analyte, such as oxygen, carbon dioxide or water vapor, through a film of interest. Typical films subjected to permeation testing are polymeric packaging films such as those constructed from low density polyethylene (LDPE), high density polyethylene (HDPE), oriented polypropylene (OPP), polyethylene terepthalate (PET), polyvinylidene chrloride (PVTDC), etc. Typically, the film to be tested is positioned within a test chamber to sealing separate the chamber into first and second cells. The first cell (commonly referenced as the sensing cell) is flushed with an inert gas to remove any target analyte from the cell and the second cell (commonly referenced as the analyte cell) filled with a gas containing a known concentration of the target analyte. A sensor for the target analyte detects the presence of target analyte that has migrated into the sensing cell from the analyte cell through the film.
Permeation instruments typically employ a flow-through method or an accumulation method for sensing the presence of target analyte in the sensing cell. Briefly, the flow-through method uses an inert flushing gas to continuously pick up any target analyte that has migrated into the sensing cell and deliver it to a remote sensor. The accumulation method allows target analyte to build up in the sensing cell for an accumulation period, with the sensor either positioned within the sensing cell or the sensing cell flushed with a flushing gas after the accumulation period for delivery of accumulated target analyte to a remote sensor.
Both methods require precision in timing the opening and closing of fluid flows through the instrument, as well as opening and closing access to the analyte sensor. In addition, when sensing cell is sealed to fluid flow during a testing period, such as occurs with the accumulation technique, the instrument is relying solely upon diffusion to move analyte molecules within the sensing cell into sensing contact with the sensor, and the valving system should not seal the sensing cell too far upstream or downstream from the cell as this causes an increase in the effective volume of the sensing cell, thereby reducing the responsiveness and accuracy of the instrument.
Accordingly, a substantial need exists for a valving system for permeation testing instruments capable of reliably and consistently opening and closing the various cells within the instrument to fluid flow with a limited increase in the effective volume of the sensing cell of the instrument.